Field of the Invention
The present invention relates to a spot shape detection apparatus capable of detecting an accurate spot shape of a laser beam.
Description of the Related Art
A wafer having a plurality of devices such as integrated circuits (ICs), large-scale integrations (LSIs), or light emitting diodes (LEDs) formed on its front surface partitioned by division lines (streets) is divided into individual devices by a laser processing apparatus for performing ablation by applying to the division lines a laser beam of such a wavelength as to be absorbed in the wafer, and the devices are used for electric apparatuses such as mobile phones, personal computers, liquid crystal television (TV) sets, illumination apparatuses and the like (see, for example, Japanese Patent Laid-Open No. 1998-305420).
In addition, there has also been proposed a laser processing technology in which a laser beam of such a wavelength as to be transmitted through a wafer is applied to the wafer along division lines, with the focal point of the laser beam positioned inside the wafer, to form modified layers along the division lines, and an external force is exerted on the wafer to divide the wafer into individual devices, and this technology is put to practical use in the field of laser processing apparatus (see, for example, Japanese Patent No. 3408805).
Besides, the spot shape of the laser beam oscillated by a laser oscillator constituting the above-mentioned laser processing apparatus has an influence on the quality of processing. In order to secure quality upon laser processing, therefore, the laser beam should be evaluated either by use of the spot shape of the laser beam applied to the wafer or by calculating an M2 factor, which is an index for evaluating the laser beam on the basis of the beam diameter calculated from the spot shape. Heretofore, some apparatuses for detecting a spot shape have been proposed (see, for example, Japanese Patent Laid-Open No. 2013-022634, Japanese Patent Laid-Open No. 2013-151002, and Japanese Patent No. 5726999). Note that the M2 factor is an index value indicative of how many times the beam diameter obtained when a laser beam is focused and converged to a diffraction limit is as large as the beam diameter obtained when an ideal Gaussian beam is converged to a diffraction limit. Therefore, the index value is represented by a ratio of not less than 1 (one) to the ideal Gaussian beam, and the M2 factor in the case where the laser beam coincides with an ideal Gaussian beam is 1 (one).